Counter-force motivated back rest



Feb. 18, 1969 G. N. I EvEsQUE ETAl. 3,427,755

M COUNTER-FORCE MOTIVATED BACK REST I Filed Dec. 2o, 1965 sheet of s j@Off f 4"/ f O i C O JJ /Zf y [ZZ s m f fi y ff N 7 l l ff W4@ y; f4 @Mwg' XZ L 'vw It f/f VW' f1 f"w v i! AMl l ma /g/ l @nu I A 1 l i I g I AI /J l :g u//r I I ff 6g l @4 l J/ .ff l v y I I L l Feb. 18, 1969 NLEVESQUE ET Al. 3,427,755

COUNTER-FORCE MOTIVATED BACK REST Sheet Filed Dec. 20, 1965 G. N.LEvEsQuE ET AL 3,427,755

COUNTER-FORCE MOTIVATED BACK REST Feb. 18, 1969 Filed Dec.

Sheet 3 of' 3 Nw L nnlw l w@ J, Q we ed@ e United States Patent O3,427,755 COUNTER-FORCE MOTIVATED BACK REST George Napoleon Levesque andPaul Eugene Taylor,

Warwick, and Neil Ide Barney, Riverside, R.I., assignors to Brown &Sharpe Manufacturing Company, North Kingstown, RJ., a corporation ofRhode Island Filed Dec. 20, 1965, Ser. No. 515,062 U.S. Cl. 51--105 10Claims Int. Cl. B24b 5 00, 17/00, 49/00 ABSTRACT OF THE DISCLOSURE Acounter-force motivated back rest for a cylindrical grinding machine,having a grinding wheel assembly including a grinding wheel, a grindingwheel support and power means for driving the grinding wheel, arotatable work support, and means for relatively moving said supports toadvance the grinding wheel against the work, which comprises a supporton which the -back rest is movable against the rotating work in adirection opposed to the force of the grinding wheel, means for applyingpressure to force said back rest against the work, a device for sensingand for signalling variable loading stresses within said grinding wheelassembly produced by engagement of the grinding wheel with the work, anda control device for controlling the pressure exterted against the backrest responsive to said signalled unbalance of pressure for regulatingsaid back rest applied pressure to apply a counter-force against thework piece proportionate to said pressure unbalance.

In a preferred form of the invention a device is provided to sense andto signal any unbalance of pressure in opposed hydrostatic pressurepockets corresponding with the engaging force of the grinding wheelagainst the workpiece, a device for signalling the force applied toforce the back rest against the work, and a servo-valve for controllingthe force applied to force the back rest against the work, including adevice controlled by a signalled increase of hydrostatic pressureunbalance to open said valve and by the signalled increase of the forceapplied to force the back rest against the work to close sai-d valve,thereby moving said valve to a position in which said signals are inbalance.

The present invention relates to an improved back rest mechanism forsupporting a rotating work piece in a cylindrical grinding machine, andmore particularly to a device for controlling the back rest to apply acounterforce to the rotating work piece which is equal and opposite tothe force exerted by the grinding wheel against the work.

Certain features of the illustrated construction which relatespecifically to a hydrostatic bearing load signalling device form thesubject matter of co-pending application Ser. 446,615 filed Apr. 8,1965, now issued as Patent No. 3,365,711 dated lan. 23, 1968 forHydrostatic Bearing Load Signalling Device. Claims to such features havefor this reason been omitted from the present application.

The back rest, which provides support for the rotating work piece heldybetween centers against the thrust of a relatively advancing grindingwheel, is normally adjusted to a position in which the rotating workpiece is solidly supported against the grinding wheel to avoiddistortions or dislocations of the work. Devices heretofore devised,have been found deficient in that they do not adequately compensate forthe variations in grinding force generated by changes in the rate ofstock removal as the work piece rotates and is reduced in diameterduring grinding, with 3,427,755 Patented Feb. 18, 1969 ICC the resultthat the grinding wheel must be fed slowly to obtain satisfactoryroundness and concentricity.

In accordance with the present invention it is proposed to press theback rest continuously against the rotating work piece at a pressure,which is controlled to cause the back rest to exert a counter-forceagainst the work piece which is always equal and opposite to the forceexerted by the grinding wheel against the work piece.

It is an object of the invention to provide a signal controlled backrest counter-force balancing device, which will ydetect and signal theforce exerted against the workpiece by the grinding wheel, will cause acounter force to be applied against the workpiece in an oppositedirection, and will by a comparison of the opposing forces regulate thecounter force applied to maintain a balance Ibetween said forces.

More specically, it is an object of the invention to provide a back restcounter-force balancing device adapted to be controlled by the signalledunbalance of a hydrostatic spindle bearing to engage the back restagainst the workpiece with a force proportioned to and opposite to theforce exerted by the grinding wheel against the work.

With these and other objects in View as may hereinafter appear, theseveral features of the invention consist in the devices, combinationsand arrangement of parts hereinafter described and claimed, whichtogether with the advantages to be obtained thereby, will be readilyapparent to one skilled in the art from the following description takenin connection with the accompanying drawings, in which:

FIG. 1 is a somewhat fgragmentary View of a cylindrical grinding machinein side elevation with parts broken away and sectioned illustrating anow preferred embodiment of the invention;

FIG. 2 is a view in side elevation on an enlarged scale of the work backrest assembly shown in FIG. 1;

FIG. 3 is a plan view of the back rest assembly shown in FIG. 2;

FIG. 4 is a diagrammatic view of the signal controlled apparatus forcontrolling the work back rest of a cylindrical grinding machine toexert a balancing counter-force against the work; and

FIG. 5 is an electrical diagram of the amplifier unit employed foramplifying each of the grinding wheel force and back rest counter forcemeasuring signals.

The invention is herein disclosed in a preferred form as embodied in acylindrical grinding machine of the general type comprising a grindingwheel and spindle rotatably supported on a slide support, movable withrelation to a workpiece rotatably supported on centers on a transverselymovable work support. The workpiece is further supported against theforce exterted by the grinding wheel during a grinding operation bymeans of a back rest which may be engaged against the opposite side ofthe workpiece to resist the thrust of the grinding wheel.

Referring more particularly to the drawings, a grinding machine is showncomprising a base 20 having mounted thereon a longitudinally movablework support 22 and a grinding wheel slide 24 movable in a forward andback direction with relation to a cylindrical workpiece, for example,the workpiece 26, supported between a headstock center 28 and afootstock center 30 carried on the work support 22 (see FIG. 4). Theworkpiece 26 is connected by means of a head stock drive pin 32 and workdrive dog 34 to be driven from a motor 36 through connections not shownsince this mechanism is well known and forms specifically no part of thepresent invention.

The grinding wheel assembly of the machine comprises a grinding wheel 38and its supporting spindle 40 which is mounted in bearings generallyindicated at 42 (see FIG. 4) having opposed hydrostatic uid bearingpockets which provide fluid cushions or pads for the rotating spindle,two such opposed hydrostatic fluid bearing pockets 43 and 44 beingspecificially illustrated. The grinding wheel 38 and spindle 40 arecontinuously driven from a mOtor 46 mounted on the grinding wheel slide24 through connections which comprise a drive belt 48 passing around amotor pulley 50 and a pulley 52 attached to the grinding wheel spindle40. In the illustrated construction the grinding wheel assembly is movedmanually to engage the grinding wheel 38 against a workpiece 26 by meansof a hand wheel 56 supported with relation to a drive shaft 58 rotatablymounted in bearings 60 and 62 in the machine base 20. The shaft 58 isconnected by gears 64 and 66 with a screw 68 which is threaded into anut 70 connected to the wheel slide 24. The screw 68 is supported torotate and is fixed against endwise movement in bearings 72 and 74 inthe base 20. The handwheel 56, which is rotatably supported on driveshaft 58, is adjustably connected to turn therewith by connections whichinclude a gear 76 keyed to shaft 58 and a pinion 78 carried on handwheel56 for engagement with said gear 76. In accordance with a usual grindingmachine operating procedure, the handwheel 56 and shaft S8 connectedthereto are turned counter-clockwise to bring the grinding wheel forwardagainst the work, and the feed of the grinding wheel is thereaftercontinued, so that the wheel removes stock until a stop dog (not shown)on the handwheel engages the size stop 80 which controls the nish sizeof the work.

The grinding wheel handwheel 56 also operates a grind-load switch 82hereinafter to be more fully described. For the retracted position ofthe grinding wheel 38 and slide 24 illustrated in FIG. 1 the camactuator 84 at the bottom of the handwheel presses the grind-load limitswitch 82 to the load position. When the cam actuator 84 leaves thelimit switch upon rotation of the handwheel to advance the grindingwheel, the switch goes t the grind position and the work supporting backrest hereinafter described moves against the work.

The signal controlled back rest counter-force balancing device whichforms more particularly the subject matter of the present inventioncomprises a work back rest assembly 86 (see FIGS. 2 and 3) consisting ofa generally L shaped fra-me 88 having along one bottom edge an undercutdownwardly extending lip 90 and along the opposite bottom edge clamps 92for securing the back rest assembly to the dove-tail edge clampingsurfaces 94, 96 of the work support 22. A back rest element consistingof a at block 98 is mounted for forward and back movement with relationto the housing 88 on two reeds 100 and 102, of which the reed 100 issecured to lugs 104 on the front edge of the housing and supports thefront end of the block 98, and the second reed 102 is connected betweenthe upward extension of the L shaped frame 88 and the rear end of theblock 98. A cover plate 106 adjustably secured to the block 98 by meansof a clamping bolt 108 carries at its forward end two spaced apartdownward extensions 110 and 112 between which is mounted a rockersupport 114 for two work piece engaging rolls 116, 118. The rockersupport is mounted intermediate its length on a pivot pin 120, and isformed above and below this pivot point with two semi-circular pockets122 to receive the respective rolls 116 and 118. It will be noted thatthe path of Imovement of the back rest element comprising the block 98and cover plate 106 slopes upwardly in a plane passing through therotational work axis along the line of thrust of the grinding wheelwhich is the resultant of radial and tangential forces exerted by thegrinding wheel. The support rolls are located, one above and one belowthis plane.

The back rest element is advanced from a normally retracted disengagedWork loading position, as shown in FIGS. 2 and 3 by means ofhydraulically operated connections which comprise a bellows typeoperator including a bellows 126 with a recessed cap 128 into which isfitted a short plunger 130 connected at its upper end with the lateralextension of an elbow shaped lever 132 which is mounted on a transversepivot 134 on the back rest frame 88. The lever 132 extends upwardlyadjacent the rear face of the reed 100, and with the lateral extensionthereof overlying the bellows type operator comprising the bellows 126and cap 128. When the bellows 126 and cap 128 are extended upwardly thelever 132 is rocked clockwise from the position shown in FIG. 2 toengage and to advance the back rest unit against the work.

Oil under pressure is supplied to the lbellows 126 through supplyconnections which include an oil reservoir 136 from which oil is drawnby a pump 138 and conduit 140 at supply pressure to the hydrostaticpressure pockets 43, 44, and to a servo-valve 142, and thence throughconduit 144 to the bellows 126. The servo-valve 142, diagrammaticallyshown in FIG. 4 consists of a valve housing 145 within which issupported a valve stem 146 having two disc shaped spools 148 and 150 andan intervening land 152. Upward movement of the valve stem 146 opens thepressure conduit 140 to the land 152 past spool 148 and admits fluidunder pressure to the conduit 144 and to the bellows 126. An exhaustconduit 154 from the valve housing is shut off by spool 150. Downwardmovement of the valve stem 146 causes the spool 148 to shut olf thesupply conduit 140 and opens a connection to exhaust conduit 154.Operation of the valve stem 146 is controlled by two oppositely orientedmagnets 158 and 160 and an armature 162 mounted to turn on a pivot 164and connected at its other end to the valve stem 146. Whenever thegrinding wheel 38 is engaged against a workpiece, exerting forcethereon, a signal in the form of an electric current is transmitted to awire coil 166 on the armature 162 in a direction to move the armatureclockwise, moving the valve stern and spool 148 upwardly to supply oilunder pressure to the bellows 126, and to move the back rest element 98against the work. A signalled indication of any force exerted againstthe workpiece by the back rest element 98 is similarly provided in theform of an electric current which passes in reverse direction around asecond wire coil 168 on the armature 162, biasing the armature in acounter-clockwise direction which will olfset the effect of the grindingwheel force generated signal.

In the illustrated form of the invention, an extremely sensitive andaccurate signalled indication is provided of any force which may bebrought to bear against a workpiece when the grinding wheel comes intogrinding contact therewith, by means of a differential hydrostaticApocket pressure sensing device, which acts, in combination withsuitable signal amplifying devices, to detect and signal instantly anyvariation in the loading of the hydrostatically supported grinding wheelspindle. The pressure differential in the hydrostatic spindle bearingpockets 43, 44 induced by the loading of the grinding wheel against theworkpiece is picked up by a hydraulic-electrical transducer 170, theelectrical signal is substantially amplified in an electrical amplifier172, and the amplified signal is fed to the armature coil 166 of theservo-valve 142 above referred to. The transducer 170 consists of anarmature 173 connected by means of a reed 174 to swing about a xed pivot176. The armature is biased in one direction by a bellows 178 to whichfluid under pressure is supplied by a fluid ypressure sensing conduit180 connected with the hydrostatic lpocket 43, and in the oppositedirection by a bellows 182 connected by a fluid pressure sensing conduit184 with the hydrostatic pocket 44.

Any movement transmitted to the armature 173 of the differentialpressure sensing device illustrated in FIG. 4 is picked up electricallyby a pair of transducer coils 186, 188 mounted on E-cores 190, 192located at opposite sides of a metallically coated portion of thearmature 173, and a voltage dividing network consisting of resistancesR1 and R3 on which there is impressed a voltage which may be in theorder of 3.5 volts supplied by wires 194, 196

from the secondary of a transformer forming part of a conventionalamplifier generally indicated at 172 and hereafter more fully describedin connection with FIG. 5 of the drawings. The resistances R1 and R3 areconnected to ground by a wire G. Any movement of the transducer armature173 toward one or the other of said E-cores 190, 192 causes a voltageoutput to appear between the ground wire G and the junction I betweenthe transducer coils 186, 188. This output voltage is carried to gaincontrol R140, and the adjustable portion of this output voltage ispicked off and applied to the input line 202 of the amplifier 172, theamplified output from which is transmitted through connections 204, 206through a visual meter 208 having a pointer 210 which indicates both thedirection and extent of the movement impressed on the armature 173 bythe differential pressure sensing device above described. The wires 204,206 are also connected through a grind-load switch 82 with the armaturecoil 166 of the servo-valve controller.

FIG. 5 is a schematic diagram of the electronic amplifier unit 172. Thedevice is powered by a well regulated direct current source representedby a battery 211 having a connection 213 with the primary TIP of atransformer TI.

Transistors Q3 and Q4 form an oscillator 214 which generates about 6volts alternating voltage in the transformer primary TIP.

The oscillator transformer primary TIP directly supplies a referencevoltage to a ring connected demodulator bridge 215. I

The oscillator transformer secondary winding TIS supplies about 31.5volts excitation through connections 194, 196 across the two transducercoils 186 and 188 and also to the voltage dividing network R1, R3.

As the transducer armature 173 is moved between the two E-cores avoltage output appears between the ground point G and the junction I,between coils 186 and 188. This output voltage is applied through thecoupling capacitor C to a conventional audio frequency amplifiercomprising transistors Q1 and Q2 which strengthens the signal andthrough transformer T2 applies the amplified signal to the demodulatorbridge 215. The demodulated output is fed to the meter 208.

The visual indicating meter 208 for the amplifier 172 may be arbitrarilyadjusted to a zero setting when the grinding wheel and spindle 40 areidling by a manual adjustment of the no load position of the transducerarmature 173 relative to the E cores 190, 192. As shown in FIG. 4 thebellows 182 is seated against an adjustable setscrew 217 which isadjusted to move the bellows 182 toward or away from the bellows 17S.

A signalled indication is provided of the hydraulic pressure built up inthe bellows 126, which is proportional to the counter-force exerted bythe back rest rolls 116, 118 against a workpiece 26, and is well adaptedto be continuously compared with the signalled indication of the forceexerted by the grinding wheel against the workpiece to control theoperation of the servo-valve 142, so that the back rest will exert acontinuously balancing counter-force against the rotating workpiece. Thehydraulic pressure in the back rest actuating bellows 126 is picked upby a hydraulic-electrical transducer 216, the electrical signal producedbeing amplified by an amplifier 218 and fed to the armature coil 168 ofthe servo-valve 142. The transducer consists of an armature 220connected by means of a reed 222 to swing about a fixed pivot 224. Thearmature 220 is biased in one direction by a bellows 226 to which ffuidpressure is supplied by a fluid pressure sensing conduit 228 connectedwith the bellows 126, and in the opposite direction by means of a coiledspring 230 seated against an adjusting screw 232. A pair of transducercoils 234 and 236 mounted on E-cores 238, 240 located at opposite sidesof the armature are connected through a voltage dividing networkincluding resistances Rla and R3a and a gain control includingresistance R140a to the amplifier 218 and a visual meter 244, whichparts are exact duplicates of the voltage net work, gain control,amplifier 172 and visual meter 208 above described. Output connections246 and 248 from the amplifier 218 and visual meter 244 are connectedthrough the grind-load switch 82 with the armature coil 168 of theservo-valve controller.

The illustrated signal controlled apparatus for actuating the back restof a cylindrical grinding machine to exert a counter-force against thework which will balance the force of the grinding wheel operates in thefollowing manner:

Transducer 170 yields an electrical signal which is proportional to theforce of the grinding wheel 38 against the work. This signal is sent tothe servo-valve 142 via amplifier 172, and operates the valve in such away as to open the pressure line to the work back rest bellows 126.Transducer 216 yields an electrical signal which is proportional to thecounter-force exerted by the work back rest rollers 116 and 118 againstthe workpiece. This signal, via amplifier 218, closes the pressure line140, 144 to the work support bellows 126. When the electrical signalsfrom each transducer are now balanced, the force on the workpiece fromthe grinding wheel 38 will be balanced by the force of the work support.

Initially, with the grinding wheel spindle idling, the amplifier 172 isadjusted to give a zero reading on its meter 208 indicating a zerosignal. Next, the work back rest 98 is moved forwardly, so that the workengaging rollers 116, 118 make light contact with the workpiece by anadjustment of the work back rest bellows counter-force indicatingtransducer 216. This adjustment is effected by means of the adjustingscrew 232, which adjusts the effective strength of spring 230 againstthe back rest bellows 126 pressure sensing conduit 228 and bellows 226.Gain control Ra is now adjusted to provide a zero reading of meter 244.These adjustments establish a nominally zero force between the back rest98 including rollers 116, 118 and the workpiece, and between thegrinding wheel 38 and workpiece for this idle condition.

As the grinding wheel now moves against the work, a differential oilpressure is set up in the wheel spindle bearing pockets 43, 44proportional to the force of the wheel against the workpiece. Thisdifferential pressure, is transmitted to the bellows 178, 182 oftransducer 170. This differential causes the armature 173 to move whichupsets the balance between the coils 186, 188 generating an electricsignal. It is this signal which is passed on to amplifier 172, and thento the servo-valve 142, causing the winding 166 to be energized openingthe servo-valve iiuid power connection to the back rest bellows 126. Theback rest rollers 116, 118 are pressed against the workpiece exerting acounter-force which tends to exceed the force exerted by the grindingwheel. The pressure rise within the back rest bellows 126 causes thearmature 220 of transducer 216 to move against the pressure of spring230 upsetting the electrical balance in the coils 234, 236 to produce anelectrical signal which is amplified and applied to energize armaturecoil 168 tending to shift the servovalve 142 to its closed position.

From the foregoing, it will be evident that the signals from the twoamplifiers are proportionate in strength to the force and counter-forceexerted respectively by the grinding wheel 38 and back rest rollers 116,118 against the workpiece, and further that these opposed signals actingupon the :armature controller 162 of the servovalve 142 will tend toproduce a stabilized relationship between the force and counter-forcedirected against the workpiece with the illustrated construction it isnecessary only to balance the signals produced by the two amplifiers 172and 218 in order to bring these forces into exact balance as, forexample, by an adjustment of the gain controls R140 and R140a of therespective amplifiers. The gain control of the amplifier 172 is set at aconvenient point to have as much response as practicable withoutinstability. The gain control R140 is now manipulated to balance the twosignals so that a dial indicator 250 located near the point of grindingshows no deiiection of the work.

When the grinding Wheel is moved away from the workpiece at the finishof the grinding process, as for example, by the return of the hand wheelS6 to the initial load position, the dog 84 on the hand wheel willreturn the work-load switch 82 to its load or off position so that theload bias 252 causes the servo-valve 142 to close its pressure port andopen its tank port. Oil pressure will now be removed trom the back restbellows 126 enabling the back rest rollers 116, 118 to back away fromthe workpiece also.

It will be understood that while a presently preferred embodiment of theinvention is here shown comprising a device for sensing and forsignalling an unbalance in the pressure pockets of a hydrostatic spindlebearing produced by variations in the force of grinding, and includingvalve means controlled by the signalled unbalance ofset by a signalledback rest applied pressure for applying a controlled hydraulic pressureagainst the back rest to move said back rest against the workpiece withan equal and opposite counter-force, the invention in its broaderaspects envisions the use of any suitable strain sensing device adaptedto produce a signal proportionate to said applied grinding force, and asuitable control device which may well include suitable hydraulic valvemechanism actuated by an amplified signal of any such strain forcontrolling the pressure applied to cause the back rest to exert acounter-force equal and opposite to the force of the grinding againstthe work.

The invention having been described what is claimed 1. A counter-forcemotivated back rest for a cylindrical grinding machine, having agrinding wheel assembly including a grinding wheel, a grinding wheelsupport and power means for driving said grinding wheel, a rotatablework support and means for relatively moving said supports to advancethe grinding wheel relative to the Work support, having in combination,a support on which the back rest is movable toward the work support in adirection opposed to the force of the grinding wheel, means for applyingpressure to force said back rest toward the work support, a device forsensing and for signalling variable unbalance producing loading stresseswithin said grinding wheel assembly during said advance of `the grindingwheel, and a control device for controlling the pressure exerted againstthe back rest responsive to said signalled unbalance producing loadingstresses for regulating said back rest applied pressure to apply acounterforce against the force of the grinding wheel proportionate tosaid unbalance producing loading stresses within said grinding wheelassembly.

2. The combination of claim 1 in which said control device comprises avalve mechanism operable for controlling the pressure applied to saidback rest, and a device actuated by the signalled unbalance producingloading stress within said grinding wheel assembly for operating saidvalve mechanism to apply a counter-force through said back restproportionate to said signalled unbalance producing loading stress.

3. The combination of claim 1 in which there is provided means forsensing and for signalling variable pressures applied to said back rest,a device for varying said applied pressure, and in which said controldevice comprises means actuated by said signalled grinding wheelunbalance producing loading stress to increase said applied pressure,and by a back rest applied pressure originated signal to decrease saidapplied pressure, thereby to proportion said back rest applied pressureto said grinding wheel loading stress.

4. The combination of claim 3 for a grinding machine provided withhydrostatic spindle bearings including opposed hydrostatic pressurepockets, which comprises a device to sense and to signal any unbalanceof pressure in said opposed hydrostatic pressure pockets correspondingwith the work engaging force of the grinding wheel, means for applyinghydraulic pressure to force the back rest in a direction opposed to theforce of the grinding wheel, a signalling device for signalling thelevel of said back rest applied hydraulic pressure, and a servo-valvefor controlling the supply of hydraulic pressure to the back rest,including a device controlled by a signalled increase of unbalance ofpressure in said opposed hydrostatic pressure pockets to open said valveand by a signalled increase in the level of said back rest appliedhydraulic pressure to close said valve, thereby moving said valvecontrol valve to a position in which said signals are in balance.

5. The combination of calim 4 in which said device for signalling anyunbalance of pressure in said opposed hydrostatic pressure pocketscomprises a hydraulic-electrical transducer and an electrical signalamplifying device, said device for signalling the level of said backrest applied hydraulic pressure comprises a hydraulic-electricaltransducer and an electrical signal amplifying device, and saidservo-valve comprises a valve controller connected to be urged inopposite directions between open and shut-off positions by saidrespective amplified electrical signals.

6. The combination of claim 5 in which each said electrical amplifyingdevice includes a visual signal strength indicating means providingzero, plus-and-minus readings and a gain control device adjustable toobtain an adjusted zero reading of said visual indicating means, for anunstressed grinding wheel idling position.

7. In a cylindrical grinding machine, having a grinding wheel assemblyincluding a grinding wheel, a grinding wheel spindle, hydrostaticbearings including opposed hydrostatic uid pressure pockets for saidspindle, a grinding Wheel support, and power means for driving saidgrinding wheel spindle, a work support including a rotative work holder,and means relatively positioning said supports for grinding, thecombination of a work back rest movable in a direction opposed to thepositioning movement of the grinding wheel, means applying a force topress the back rest in said direction opposed to the grinding wheel, anda signal controlled back rest counterforce balancing device whichcomprises a signalling device connected with said opposed hydrostaticfluid pressure pockets to detect and signal any unbalance of pressure insaid hydrostatic bearing induced by the Work engaging force of thegrinding wheel and a control device for controlling the pressure exertedagainst the back rest responsive to any unbalance of pressure in saidhydrostatic bearings for regulating said back rest applied pressure toapply a counter-force proportionate to said pressure unbalance in saidhydrostatic bearings.

8. The combination of claim 7 which includes an unbalance sensitivehydrostatic iluid pressure pocket signal producing transducer andamplifier, means for applying hydraulic pressure to force the back restin said direction opposed to the grinding wheel, a back rest hydraulicpressure sensitive signal producing transducer and ampliiier, a backrest pressure control valve, and a valve operator acted upon by ahydrostatic fluid pressure pocket unbalance signal to open said valve,and 4by a said back rest pressure to close said valve thereby to producea balance position of said valve operator and valve controlled thereby.

9. The combination of claim 8 in which said back rest hydraulic pressuresensitive signal producing transducer comprises an armature, a hydraulicpressure sensing device acting upon said armature in one direction, andmeans exerting pressure upon said armature in the opposite directionadjustable to initially position the back rest in an idling light workcontact position.

10. The combination of claim 9, in which means are provided foramplifying each of said signals including means to adjust theamplification of one said signal rela.-

tively to the other said signal to provide a counter balancing force ofsaid back rest equal and opposite to the force of the grinding wheel.

References Cited UNITED STATES PATENTS 2,099,161 11/1937 De Leeuw 51-238X 2,141,596 12/1938 Crompton 51-238 Lillie 51-165 Price et al 51-105Price 51-105 Kuniholm 51-165 LESTER M. SWINGLE, Primary Examiner.

U.S. Cl. X.R.

